Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Oct 14, 2022 at 04:31:37PM +0200, Ard Biesheuvel wrote:
> +       if (slot != -1) {
> +               /*
> +                * Use release semantics to ensure that ghes_estatus_cached()
> +                * running on another CPU will see the updated cache fields if
> +                * it can see the new value of the pointer.
> +                */
> +               victim = xchg_release(ghes_estatus_caches + slot,
> +                                     RCU_INITIALIZER(new_cache));
> +
> +               /*
> +                * At this point, victim may point to a cached item different
> +                * from the one based on which we selected the slot. Instead of
> +                * going to the loop again to pick another slot, let's just
> +                * drop the other item anyway: this may cause a false cache
> +                * miss later on, but that won't cause any problems.
> +                */
> +               if (victim) {
> +                       call_rcu(&rcu_dereference(victim)->rcu,
> +                                ghes_estatus_cache_rcu_free);
		}

I think you can use unrcu_pointer() here instead, there should not be a
data dependency since the ->rcu member itself should be otherwise unused
(and if it were, we wouldn't care about its previous content anyway).

But only Alpha cares about that distinction anyway, so *shrug*.

While I much like the xchg() variant; I still don't really fancy the
verbage the sparse nonsense makes us do.

		victim = xchg_release(&ghes_estatus_caches[slot], new_cache);
		if (victim)
			call_rcu(&victim->rcu, ghes_estatus_cache_rcu_free);

is much nicer code.

Over all; I'd simply ignore sparse (I often do).

> +       }
>         rcu_read_unlock();
>  }